1. Field of the Invention
The present invention relates to a seat position sensor which is served for detecting a position of a seat of an automobile or the like and for controlling an inflation of an airbag or the like.
2. Description of the Related Art
To explain a conventional seat position sensor using drawings, FIG. 10 is a perspective view showing the conventional seat position sensor in a mounted state, FIG. 11 is an enlarged perspective view showing the conventional seat position sensor in the mounted state, FIG. 12 is a cross-sectional view of an essential part of the conventional seat position sensor in the mounted state, and FIG. 13 is a cross-sectional view of the essential part of the conventional seat position sensor.
To explain the conventional seat position sensor in conjunction with FIG. 10 to FIG. 13, each first rail 51 is constituted by bending a metal plate in an approximately U shape and a pair of these first rails 51 are fixed to both sides of a lower surface of a seat 60. A shielding plate 54 is constituted by bending a metal plate in an L shape and includes an elongated planer base 54a and a side wall 54b which is formed by being bent and extended downwardly from an end of the base 54a, wherein the base 54a is fixed to the first rail 51 and the shielding plate 54 is slidably moved together with the first rail 51.
A second rail 52 is constituted by bending a metal sheet in an approximately U shape and is combined with the first rail 51 such that the second rail 52 forms an inversely projecting shape as shown in FIG. 12. The second rail 52 is fixed to an indoor floor surface of a vehicle using a holding strut 53 bent in an L shape.
A seat position sensor 55 is formed of a molded product made of synthetic resin and includes a base body 55a which is formed in a U shape, a rectangular magnet 56 which is embedded into a first side wall 55b of the base body 55a, and a rectangular Hall IC 57 which is embedded into a second side wall 55c which is formed in an opposed manner with respect to the first side wall 55b and faces the magnet 56 in an opposed manner. As the magnet 56, a magnet having a strong magnetic force such as samarium or cobalt can be used. The seat position sensor 55 having such a constitution is mounted on the second rail 52 by means of a stay 58.
In the state that seat position sensor 55 is mounted, the side wall 54b of the shielding plate 54 can enter into or exit from a space 55d defined between the first and second side walls 55b, 55c, wherein when the shielding plate 54 is positioned inside the space 55d, a magnetic field generated by the magnet 56 is shielded by the shielding plate 54 so that the magnetic flux density which the Hall IC 57 receives is lowered thus generating the changeover of an output of the Hall IC 57.
To explain the manner of operation of the conventional seat position sensor 55 having the abovementioned constitution and the mounting state, when the seat 60 is moved, the shielding plate 54 enters into or exits from the space 55d of the seat position sensor 55 so that the changeover of the output of the Hall IC 57 is performed. Due to this changeover of the signal, a gas flow-out quantity at the time of inflation of an airbag (not shown in the drawing), for example, is changed so that an airbag is adjusted.
In the conventional seat position sensor which has the abovementioned constitution and performs the abovementioned operation, as explained above, when the shielding plate 54 is positioned inside the space 55d, the flux density which the Hall IC 57 receives is lowered. However, since a gap is formed between the side wall 54b of the shielding plate 54 and the space 55d and the magnetism leaks from this gap. Here, the Hall ICs 57 have considerable irregularities with respect to sensitivity, that is, the Hall ICs 57 have the considerable irregularities with respect to the magnetic flux density at the time of changeover so that when the Hall IC having a favorable sensitivity (the Hall IC which is changed over with small magnetic flux density) is used, there arises a problem that the changeover of the output of the Hall IC is not performed due to the influence of the leaked magnetic flux. Although it may be possible to use the Hall IC having a poor sensitivity (the Hall IC which is changed over with large magnetic flux density) to cope with such a problem, there arises a problem that the yield rate is drastically decreased.
Accordingly, it is an object of the present invention to provide a seat position sensor which can ensure a high yield rate by reducing influence of leaked magnetic flux using a bias magnet, by ensuring changeover of a magnetoelectric converting element by controlling a magnetic flux density and further by using a magnetoelectric converting element in a wide range.
According to a first aspect of the present invention to solve the abovementioned problems, a seat position sensor of the present invention includes a housing, a magnetoelectric converting element which is held by the housing and performs changeover of an output at a given magnetic field intensity, and a magnet which is held by the housing at a position where the magnet faces the magnetoelectric converting element in an opposed manner, wherein a shielding plate is capable of entering into or exiting from a space defined between the magnetoelectric converting element and the magnet, and an inverse bias magnet which applies a magnetic field of an inverse direction to the magnet is provided at a side opposite to the magnet side while sandwiching the magnetoelectric converting element therebetween.
Further, according to a second aspect of the present invention to solve the abovementioned problems, the housing of the seat position sensor of the present invention includes a first side wall, a second side wall which faces the first side wall in an opposed manner, and a connection wall which connects the first with the second side walls and form the space into a recess, wherein the magnetoelectric converting element and the magnet are respectively mounted on either one of the first and the second side walls, the shielding plate is formed in a planar shape and is allowed to be disposed inside the recess from an opening side of the recess toward the connecting wall, and an end surface of the shielding plate at the connection wall side is positioned closer to the connection wall side than the magnet.
Further, according to a third aspect of the present invention to solve the abovementioned problems, the housing is arranged such that the open-side of the recess of the space of the seat position sensor of the present invention is positioned downwardly.
Further, according to a fourth aspect of the present invention to solve the abovementioned problems, the bias magnet is arranged at the first or the second side wall on which the magnetoelectric converting element of the seat position sensor of the present invention is arranged and the bias magnet, the magnetoelectric converting element and the magnet are positioned on one straight line.
Further, according to a fifth aspect of the present invention to solve the abovementioned problems, the magnetoelectric converting element of the seat position sensor of the present invention is constituted of a Hall element and a magnetization surface of the magnet is parallel to the shielding plate.
Further, according to a sixth aspect of the present invention to solve the abovementioned problems, the changeover of the output of the magnetoelectric converting element is performed only once with respect to movement in an exiting direction or in an entering direction of the shielding plate which is capable of entering into or exiting from the space of the seat position sensor of the present invention.
Further, according to a seventh aspect of the present invention to solve the abovementioned problems, the housing of the seat position sensor of the present invention is arranged at a movable member side including a seat, the shielding plate is arranged at a fixed member side which holds the movable member in a movable manner, and the housing is slidably moved along with slide movement of the movable member so as to allow the shielding plate to enter into or exit from the space.